Carbon Monoxide (CO) is one of major pollutants emitted from combustion sources during the combustion of fossil fuels. A high level of CO in the combustion exhaust is a result of incomplete oxidation of fossil fuels and a relatively low oxidizing rate of CO downstream of the combustor. Due to these factors, the amount of CO being discharged into to the ambient air may be significant. Often, the concentration of CO in the exhaust stream from gas turbines can reach 1,000-1,500 ppm during low load operations. In lean premixed hydrocarbon flames, which are very typical for operation of gas turbines, CO is particularly rapidly formed in the flame zone. The CO is oxidized to CO2 at a rate that is slower than the rate of formation of CO. Numerous catalytic processes have been developed that attempt to decrease emissions of CO from combustion sources, e.g., from gas turbine engines, by improving combustion and/or by utilizing a CO catalyst downstream of the combustion sources, e.g., in a heat recovery steam generator (HRSG) of the combined cycle power plant. Some known systems and processes for reducing CO emissions from combustion sources utilize a catalytic coating that can be coated on the walls of the gas turbine's elements before and after the combustor in order to improve flame stability and completeness of the combustion, thereby providing reduction of CO downstream of the flame.
For example, U.S. Pat. No. 5,355,668 describes a catalytic coating that is applied to at least a portion of the internal surfaces of substantially the whole flow path within a gas turbine to catalyze the combustion of the fuels in order to reduce emissions of CO at least by 15% at an equivalence (fuel to air) ratio of at least 0.8 (rich combustion). This small increase in CO reduction efficiency is achieved primarily due to the enhanced flame stabilization (as a result of the catalyst) and does not significantly reduce CO emissions to the environment, especially for gas turbines operating at low loads where CO concentration in the exhaust is generally very high. As is evident from U.S. Pat. No. 5,355,668, a reduction in the equivalence ratio below 0.8 substantially decreases the efficiency of the catalytic coating, resulting in a reduction of CO efficiency less than 15%. Currently, however, typical gas turbines require the utilization of lean combustion with fuel to air equivalence ratio below 0.6. As such, the proposed solution of U.S. Pat. No. 5,355,668 is impractical for use with such exhaust gases from lean combustion. Operation of the process of U.S. Pat. No. 5,355,668 with exhaust gases from lean combustion, e.g., less than 0.8, results in increased CO emissions, which are ultimately released to the environment.
Other processes for reducing CO emissions employ a catalytic process that reduces CO emissions by direct oxidation of CO to CO2. The reaction between CO and O2 is very slow, and the presence of catalytic materials significantly improves the rate of this reaction. As a general rule, known CO oxidizing catalysts are used at the temperatures below 800° C. (preferably below 600° C.) due to the fact that the desorption rate of oxygen from the catalytic surfaces is substantially increased with temperature, thereby reducing the availability of oxygen species at the catalyst surface for the reaction of the oxygen with CO. In U.S. Pat. No. 6,831,036, for example, the usage of the catalyst with the high oxygen storage capacity enables CO emissions to be reduced at temperatures up to 800° C. only. Other catalytic systems are oriented to react with CO at the surface of the catalyst to produce CO2. Unfortunately, the conversion of CO to CO2 at the catalyst surface occurs at a relatively slow rate and is commonly insufficient to manage CO emissions at low load operations, e.g., combustion with a fuel mixture having a fuel equivalence ratio of 0.8 or less.